It is know that amongst metals used to manufacture metal tubes, copper and copper alloys have comparatively good corrosion resistance. Suitable copper allows having excellent corrosion resistance include Cu-Sn, Cu-Ni, Cu-Al, Cu-Zn-A, Cu-Sn-Pb, and Cu-Ag. However, most of these copper allows contain large amounts of added metals. In the manufacture of tubes, working conditions for melting, casting, and plasticity become inevitably severe as compared with the case where pure copper or other copper alloy tubes containing a minor amount of other added metals are manufactured so that such alloy tubes become very expensive.
On the other hand, when pure copper or copper alloy of a lower content containing an additive such as Sn, Ni, Al, Zn, Pb, Ag or the like is used for manufacturing tube members, the tubes may be produced at a much lower cost.
A copper alloy having excellent corrosion resistance exhibits its corrosion-resistant function at only the surface portion which is in contact with liquid, gas or the like, and the remainder shares only the mechanical strength of the shaped copper alloy tube.
Accordingly, a composite tube constructed of pure copper or a copper alloy of a low additive content and wherein only the inner surface thereof is a copper alloy having excellent corrosion-resistance, functions as well as a metallic tube made entirely of corrosion-resistant materials.
Furthermore, the following methods for manufacturing composite tubes by covering the inner surface of a tube with different metal from that of the tube itself have heretofore been know:
(1) Extrusion
A composite (double) tube is obtained by employing a composite extruded billet which has previously been prepared by combining an inner layer with an outer layer, and covering the inner surface of the tube with a different metal using extrusion molding. However, this method is expensive since it involves many manufacturing steps such as preparation, extruding and further drawing of composite billets to reduce the diameter thereof to a prescribed dimension. Furthermore there is also the possibility, that where the metal covering the inner surface is a metal of low melting point, it can melt due to heat at deformation in case of extrusion molding, so that suitable composite tubes cannot be obtained. In addition to the above disadvantages, there may also be difficulties in normal extrusion, drawing and the like where there is a sufficient difference in distortion resistance between a tube body member and covering member.
(2) Inner surface plating
In this method, the covering is made by passing a plating solution of a coating metal through the inner surface of a tube for electroplating thereon. Such method is, however, unsuitable for industrial production because the controls for concentration of plating solution, current density and the like become complicated for treating a tube member in continuous lengths.
(3) Welding tube
In this method a tube member is clad with a covering material and the tube material and the covering material are subjected to seam welding to form a tubular shape. However such tube material and covering material are molten and admix with each other at the welding point so that it is difficult to effect normal welding. Moreover there are further disadvantages in than either the covering metal appears at the outer surface of the tube, or the tube body material appears at the inner surface of the tube at the seam-welded portion thereby preventing a uniform covering layer over the inner surface of the tube.